The present invention relates to a color cathode ray tube of the shadow mask type capable of reducing the doming phenomenon of a shadow mask, and more particularly to a color cathode ray tube, in which a material having a low thermal conductivity is vacuum-deposited on the main surface existing on the electron gun side of a shadow mask.
When a color cathode ray tube of the shadow mask type is operated, a shadow mask is bombarded with an electron beam. Hence, the shadow mask is heated, and thus expanded. In a case where the whole of the shadow mask is uniformly heated and expanded, the positional relation between the shadow mask and a fluorescent screen can be kept unchanged, by appropriately selecting the structure and material of a member for supporting the shadow mask. That is, it is possible to prevent the adverse effect of the thermal expansion of the shadow mask on the picture quality of a displayed image.
As is well known, the shadow mask is formed of a thin metal plate. Accordingly, in a case where a local area of the fluorescent screen becomes very bright, that is, a large current is supplied to the local area, it is impossible to dissipate a large amount of heat generated in that portion of the shadow mask which corresponds to the local area, in a short time by thermal conduction. That is, the above portion is thermally expanded in a great degree. Thus, the so-called "doming phenomenon" is generated, and color unbalance occurs. While, in a case where a local area of the fluorescent screen is very dark, that is, only a small amount of current is supplied to the local area, that portion of the shadow mask which corresponds to the local area, is thermally expanded only a little, and thus the doming effect and the color shading are hard to generate.
In order to solve the above problem, a method has been devised. In this method, as described in a Japanese Patent Publication No. 57-9,184, a heat insulating layer is formed on that surface of a shadow mask which is bombarded with an electron beam, other than electron-beam transmitting holes, and a thin metal film is formed on the heat insulating layer, to dissipate heat due to electron bombardment by thermal radiation from the thin metal film, thereby preventing the temperature of the shadow mask from rising. Thus, the color shading due to the thermal expansion of the shadow mask can be prevented.
Further, according to another conventional method, as described in Japanese Patent Publication Nos. 60-14,459 and 61-6,969, an electron reflecting layer made of an element with a high density (namely, large specific gravity) is formed on that surface of a shadow mask which exists on the electron gun side, to prevent electrons from penetrating into the interior of the shadow mask, thereby preventing the kinetic energy of each electron from being converted into thermal energy. Specifically, it is disclosed in the Japanese Patent Publication No. 60-14,459 that a solution containing a heavy metal with an atomic number more than 70 is sprayed on that surface of the shadow mask which exists on the electron gun side, while being sucked from the fluorescent screen side, to form the electron reflecting layer on that surface of the color selection electrode of the shadow mask which exists on the electron gun side. In this case, however, the solution adheres to the wall of each of electron-beam transmitting holes, since the solution is sprayed on the surface of the shadow mask from the electron gun side. Hence, the electron reflecting layer is formed on the wall of each hole, and thus halation appears on the fluorescent screen.
Further, it is disclosed in the above-referred Japanese Patent Publication No. 61-6,969 that the electron reflecting film having a thickness of about 10 .mu.m and made of an element which has a density greater than the density of a substance making the color selection electrode of the shadow mask, or a compound containing the above element, is formed on that surface of the color selection electrode which is irradiated with the electron beam. In this case, there is a fear that the shape of each of electron-beam transmitting holes of the color selection electrode is changed by the electron reflecting layer.
Further, the above-mentioned conventional, methods pay no special attention to manufacturing technology and cost, and hence it is very difficult to mass-produce a desired color cathode ray tube by these conventional methods.